Recent studies have suggested that carcinoembryonic antigen (CEA)-promoter sequences are active only in CEA-positive cells, filing in the criteria for tumor specific targeting of suicide genes. However, the present study on gene therapy of colon cancer and cell-specificity of CEA promoter, provide evidence that CEA-positive and CEA-negative cells transfected with E. coli cytosine deaminase (CD) gene under the control of CEA promotor sequence are sensitive to enzyme/pro-drug therapy with 5-fluorocytosine (5-FC). Individual clones derived from the CEA-negative cell lines: melanoma Hs294T and glioblastoma T98G after transfection with CD differed profoundly in their sensitivity to 5-FC. The IC50 values for several clones of the CEA-negative cells were almost the same as for CEA-positive colon cancer cells. Such 5-FC-sensitive clones derived from the population of CEA-negative cells, present even in small number, because of the very effective bystender effect of this enzyme/pro-drug system can cause severe problems during therapy by efficiently killing surrounding normal cells. Safety is the major issue in gene therapy. Our data suggest that the safety of gene-directed enzyme pro-drug therapy (GDEPT) with CEA promoter driven expression of therapeutic genes is not so obvious as it has originally been claimed.
Analysis of the human carcinoembryonic antigen promoter core region in colorectal carcinoma-selective cytosine deaminase gene therapy